In some diesel engines having variable-capacity turbo superchargers that adjust an inlet area (nozzle opening degree) of an exhaust turbine via an adjustable nozzle to change a supercharged pressure characteristic, an EGR operation is implemented for directing a part of an exhaust gas to the flow of intake air to reduce NOx (nitrogen oxides) emissions.
The amount of exhaust gas (exhaust reflow amount) flowing to the intake airflow by operation of the EGR is adjusted by an EGR valve. Specifically, the EGR valve is subjected to a feedback (F/B) control loop in accordance with a deviation between a target fresh air amount set on the basis of operational conditions of the diesel engine (the rotational number of the engine, the injection amount, etc.) and an actual fresh air amount detected by an air flow meter so that the target fresh air amount and the actual fresh air amount closely coincide.
If the nozzle opening degree of the turbo supercharger is changed when the EGR operation is carried out, the exhaust pressure is increased and the pressure difference between the front and rear sides of the EGR valve is increased so that the exhaust reflow amount for the same opening degree of the EGR valve is increased. As a result, the fresh air amount influences the EGR control.
On the other hand, in a diesel engine in which the EGR rate is controlled to be greater than one of a gasoline engine (for example, the EGR rate is increased so that the EGR amount and the fresh air amount are nearly equal to each other), the exhaust reflow amount is greatly varied in accordance with the variation of the EGR rate so that exhaust energy acting on an exhaust turbine greatly influences a supercharged state.
Accordingly, when both the EGR and the turbo supercharger are independently and substantially simultaneously adapted to the optimum F/B gain and subjected to the F/B control operation, a problem occurs in that the control operations interfere with each other and changes occur in the control parameters (fresh air amount and supercharging pressure).
Japanese Patent document JP-A-2001-140652 discloses a well known technique for preventing interference between EGR and turbo supercharger controls. It describes that a prohibiting area for prohibiting the F/B control operation of the turbo supercharger is set in connection with an operating area in which the EGR operation is carried out. The turbo supercharger is switched to open control in the prohibiting area.
Furthermore, Japanese Patent document JP-A-2001-003796 describes that the exhaust pressure is estimated by using a physical model. The operation amount of the EGR valve is determined in anticipation of an effect on the EGR amount by a variation in the exhaust pressure caused by the operation of the turbo supercharger.
In the well-known technique described above in JP-A-2001-140652, the turbo supercharger is switched to the open control in the F/B prohibiting area so that dispersion occurs in the supercharging pressure due to dispersion among turbo supercharger products. Therefore, as shown in FIG. 6, dispersion occurs in the EGR rate so that dispersion of NOx is intensified. Furthermore, in the F/B prohibiting area of the turbo supercharger, it is impossible to carry out such F/B adaptation such that the operational performance of an actuator for driving an adjustable nozzle is conducted at a maximum level. This lengthens a steady-state deviation attenuating time needed to converge the supercharging pressure to a target value and the steady-state deviation cannot be attenuated. As a result, the response of the supercharging pressure is delayed or the target value of the supercharging pressure cannot be achieved. Accordingly, in the diesel engine, a sufficient amount of air cannot be taken and it is required to restrict the maximum value of the injection amount in order to prevent smoke such that sufficient acceleration performance cannot be achieved.
On the other hand, in the well-known technique described in JP-A-2001-003796, the physical model for estimating the exhaust pressure is complicated and a large load is imposed on the operation processing of a microcomputer such that this technique is costly. Furthermore, since the operation amount of the EGR valve is determined in anticipation of the effect of the exhaust pressure variation on the EGR amount, the control index (fresh air amount) of the EGR is stabilized. However, the supercharging pressure cannot be stabilized because no attention is paid to the effect of the operation result of the EGR valve on the supercharging pressure.
The present invention has been implemented in view of the foregoing problems and has an object to provide a control device for a diesel engine that can avoid mutual interference between an EGR control operation and a supercharging pressure control operation, control an EGR so that the EGR satisfies a target value even when the opening degree of an adjustable nozzle is varied, and also control the supercharging pressure so that the supercharging pressure satisfies a target value even when the opening degree of an EGR valve is varied.